Composite materials, including, for example, fiber-reinforced resin materials, offer a number of advantages over conventional metal materials including high strength-to-weight ratios and good corrosion resistance. Conventional composite materials typically include glass, carbon or polyaramide fibers in woven or non-woven configurations. In the raw material stage the fibers can be preimpregnated with resin or left dry. If dry, the fibers can be infused with resin after layup on a mold surface. Heat or pressure can be applied to the resin-impregnated fibers on the mold surface to cure the resin and harden the laminate in the shape of the mold. The heat or pressure can be applied with an oven, an autoclave, a heated flat or contoured forming tool, or a combination of methods including the use of a vacuum bag.
Composite parts can be formed in the above manner on both male and female tools. With male tools, the composite plies are applied to an exterior mold surface that forms an inner mold line of the part. Adding plies to the layup on a male tool increases the thickness of the part and changes the outer mold line, but the inner mold line remains unchanged. Conversely, with female tools, the composite plies are applied to an interior mold surface that forms an outer mold line of the part. Adding plies to the layup on a female tool increases the thickness of the part and changes the inner mold line, but the outer mold line remains unchanged.
Female tools are desirable when the mating surface is located on the exterior of a part because female tools allow the outer mold line (i.e., the exterior surface) to be tightly controlled. Female tooling (also known as “outer mold line tooling”) is also desirable when making multiple parts having the same external dimensions but different thicknesses. Aircraft fuselages, for example, often have multiple frames with the same external dimensions but different thicknesses. In this situation, all of the frames can be made with a single female tool because the tool allows the thickness to vary without changing the external dimensions. If future growth of the aircraft requires further thickening of the frames, this can be achieved without changing tooling. Conversely, if male tooling were used, then a separate tool would be required for each different frame thickness.
A hollow or tubular mold is a special case of female tooling including at least one enclosed cross section. Some hollow or tubular molds can be faceted, that is, can include a combination of relatively flat wall segments and conjoining internal radii, or corner regions, that define an enclosed cross section or a cavity. In this case an inflatable mandrel, or bladder, can be positioned inside the hollow or tubular mold and can be inflated to compress the composite material against the interior surface of the mold. However, when manufacturing composite parts with hollow or tubular tooling, the composite plies sometimes “bridge” or wrinkle across corner regions of the mold surface, resulting in poor consolidation or resin richness of the composite material in corner regions, with inferior material properties, including, for example, excess material thickness and porosity.
Solid mandrels made of a material with a coefficient of thermal expansion that is higher than that of the mold or the composite material, for example, aluminum or Teflon, have been used with hollow or tubular molds to avoid corner bridging and wrinkling. However, the use of these mandrels is generally limited to mold for composite parts that have smooth, straight interior surfaces, because curves, bends or angles on the mold interior surfaces can prevent removal of the solid mandrel after the composite material has been cured.
To address this problem, mandrels have also been made from materials with a relatively high coefficient of thermal expansion that can be washed or broken out of the mold after the composite material has been cured. Such materials include eutectic salts and soluble plasters. However, these mandrels require significant investment in casting tools, drying ovens, storage racks, and the like. In addition, these materials have low tensile strength, which tends to make the mandrels fragile and requires careful handling. Furthermore, some of these materials generate environmentally hazardous waste.
Accordingly, it is desirable to provide a method and apparatus that can more evenly compress a composite material against the relatively flat wall surfaces and into the internal corner regions of a hollow or tubular mold, that can be removed from molds for composite parts that have curved or angled internal surfaces, that can be reused, and that in some instances can be relatively easily and inexpensively manufactured without generating substantial environmentally hazardous waste.